Such a converter is known from WO 03/075437 (U.S. Pat. No. 7,164,219) which documents the most relevant prior art Applicant is aware of.
Although this publication describes in detail an embodiment of such a converter without permanent magnetic members it mentions in claim 4 an interrotor (the second rotor) which is provided on one surface with permanent magnetic material and on the other surface with slots accommodating an externally accessible control winding. Page 5 line 26-page 6 line 12 describes how this configuration should result in the creation of ‘virtual magnet poles’ on the rotor surface into which the slots open, which poles can be shifted in the circumferential direction of the interrotor under the influence of control currents in the winding. The purpose thereof is understood to try to influence that part of the magnetic flux generated by the permanent magnetic members which penetrates into the stator and this stator flux must be reduced when the converter operates at higher rotational speeds. Indeed reducing the magnetic flux in the stator of a rotating machine when same operates at higher rotational speeds is a measure which is known in itself for a very long time. The way in which this is brought about according to the above described proposal is, however, not very efficient while the rotor structure necessary for implementing this proposal has several drawbacks. WO 03/075437 also observes that for creating shiftable virtual magnet poles on the surface of the interrotor (thus no material poles and no salient poles) this rotor must have a substantial smooth surface which forbids the use of slots with a substantial dimension in the circumferential direction of the interrotor. Thus any windings must be accommodated in slots in the rotor body which are in connection with the interrotor surface via narrow entry slits with a width somewhat greater than the diameter of the wire as used for the winding. As a result the finally obtained windings, lying in these slots, will be very irregular with a small filling factor and cannot be very effective.